This invention relates to ultrafine particles and a process for producing the particles.
Ultrafine particles not greater than 100 nm in diameter are considerably different from ordinary particles in various characteristics. For example, gold (Au) shows a marked depression of its melting point when its diameter is reduced to 10 nm or less. Furthermore, ultrafine particles exhibit high catalytic activity, among other characteristics, thus being materials offering new application possibilities in a variety of fields. Ultrafine particles of metals, in particular, are expected to find application in the low-temperature sintering pastes for wiring electronic devices.
However, the conventional production technology for ultrafine particles has many problems. For example, one known process comprises vaporizing a starting metal in vacuo in the presence of a minor amount of gas and recovering ultrafine particles of the metal from the gas phase. However, this process yields only a small amount of ultrafine particles per batch. Moreover, the vaporization of a metal requires a equipment such as an electron beam, plasma, laser or induction heater and, when the cost of production is taken into consideration, the process cannot be considered to be suited for mass production. Furthermore, the ultrafine particles obtainable by such a gas-phase technology have the physical drawback that they are rather ready to undergo coagulation.
In contrast to the above gas-phase technology, there is known a technology by which ultrafine particles are prepared from a liquid phase. For example, a known process comprises reducing an ammoniacal silver nitrate complex solution in a hydrophobic reactor to give ultrafine particles of silver. However, the ultrafine particles obtainable by such liquid-phase technology are also comparatively ready to undergo coagulation.
Moreover, those processes require coating with a surfactant in order that a stable dispersion may be insured by a protective colloid effect but such a method has room for improvement in the dispersion stability of particles.